Puffer circuit breakers using SF.sub.6 are well known. They include a moving assembly comprising main contact fingers, arcing contact fingers, an insulating gas blast nozzle, and a piston and cylinder arrangement. To break a circuit, the moving assembly is caused to move, thereby compressing the gas contained in the cylinder and causing it to escape through the nozzle to blast the arc which strikes between the arcing contacts thereby extinguishing it rapidly.
When a high amplitude current is to be interrupted, a large arc is created and the energy liberated may build up back pressure at the nozzle thereby slowing down the motion of the moving assembly and hindering the circuit breaking operation.
This kind of defect is remedied by increasing the energy available for driving the moving assembly, for example by using hydraulic transmission which readily enables the speed of the moving assembly to be regulated in spite of the excess drive energy available.
However, such a device is bulky and expensive; further, since little energy is needed to close the circuit through the circuit breaker compared with the energy needed to open the circuit, the circuit breaker has to respond to unbalanced drive forces which makes it difficult to construct and to adjust.
One aim of the invention is to provide a circuit breaker in which the opening speed of the moving assembly is controlled by simple means which do not need disproportionate amounts of energy for opening and for closing, but rather which tend towards similar amounts for both operations.
Another aim of the invention is to provide a circuit breaker in which the arc blasting operation is accompanied for medium to high value currents to be interrupted, by effective puffer operation.